Semiconductor lasers are the basis for optoelectronics, and also many other devices that make use of laser emissions. Laterally emitting lasers are in widespread commercial use. These type of lasers emit from the cleaved facet in the lateral direction. While high powers may be obtained, the lateral emission is inconvenient for integrating the device with other similar or different devices. Laterally emitting lasers also have elliptical beams that diverge at a greater rate along the major axis of the elliptical cross-section of the beam. The uneven divergence and shape of the elliptical beams sometimes limits the effectiveness of the laterally emitting laser devices.
Another type of semiconductor laser is the vertical cavity surface emitting laser (VCSEL). The VCSEL emits in the vertical direction. That provides advantages for the integration of the VCSEL with other like or different devices. Though VCSELs typically emit lower power than the laterally emitting lasers, the VCSELs provide a beam having a circular cross-section. The VCSELs are also very compact. A typical VCSEL is a few to ten microns in length and about 1 to 10 microns in diameter. In addition, the circularly shaped beam of a VCSEL diverges at an equal rate in all directions. Although the VCSEL usually inherently exhibits a single longitundinal mode, it is important to achieve single transverse mode operation.
To achieve single transverse mode operation in a VCSEL, several approaches have been pursued. For a conventional ion implanted structure, which has no lateral index confinement, single mode operation is achieved by controlling the size of the transverse cavity diameter. This intrinsically limits the single mode output power. Oxide confined VCSEL structures with sufficiently small aperture diameter have also demonstrated single mode operation. A small oxide aperture realizes single mode operation where higher order modes are in cut-off. However, the refractive index contrast between oxidized and unoxidized regions is so high that the diameter of the aperture should be smaller than 3 to 4 microns at an 850 nm emission wavelength. This geometry is difficult to manufacture and it results in unavoidably high electrical resistance and limited output power of less than around 1 mW. More complicated approaches for achieving single mode emission in VCSELs include surface relief etching, see, e.g. H. J. Unold et al., IEEE Journal of Selected Topics in Quantum Electronics, vol. 7, No. 2, p386, 2002; mode selective reflectivity, see, e.g., N. Ueda et al., IEICE Trans. Electron., vol. E85-C, No. 1, p64, 2002; and, an extended cavity structure which applies an additional diffraction loss on higher order modes, see, e.g., D. G. Deppe et al., Electronics Letters, vol. 33, No. 3, p211, 1997. In addition, an ion implant and oxide aperture can be combined within a single device to achieve single transverse mode operation, see E. W. Young, et al, Photon. Tech. Lett. Vol 13, p927, 2001. These approaches each deteriorate the lasing performance of the fundamental mode along with the undesirable higher order modes because the fundamental mode and the other higher order modes have a degree of spatial overlap. Introducing scattering losses in the higher order modes introduces loss to the fundamental mode.
There are Japanese patent publications directed to photonic crystal confined VCSELs, see, Japanese Patent publications H10-284806A and Japanese Patent H11-186657A. These publications show a photonic structure including a pattern of holes, and suppose the existence of a photonic bandgap at operating wavelength. Similarly to these publications, the existence of a single mode is claimed regarding a photonic structure including a pattern of holes and a defect in Song et al., Appl. Phys. Lett Vol. 80, number 21, May 27, 2002. However, the publications provide no guidance on setting device physical parameters to permit the design of VCSELs exhibiting single transverse mode operation.
To achieve coherent coupling between VCSELs in a 2D array, several strategies have been previously demonstrated. A leaky mode approach based on changes of the cavity length has been shown to provide coupling between elements, although complex fabrication processing including a re-growth step is required, see, e.g. D. Serkland, K. D. Choquette, et al., Appl. Phys. Lett. 75, 3754 (1999). Deep etching the array pixels combined with a “checker-board” phase matching layers has demonstrated a coupled lowest order array mode, but this also involves complex fabrication procedures, see, e.g., M. Warren, et al., Appl. Phys. Lett. 61, 1484 (1992). The objective to all of these approaches is to establish the lowest order supermode, which has high power in the central far-field lobe; achieving this objective will enable the development of very high power (>100 mW) single mode VCSEL sources. There remains a need for improved VCSEL structures which enable single transverse mode operation and/or transverse coupling.